Optical And Non-Optical Sensor Tracking of a Robotically Controlled Instrument

ABSTRACT

Surgical systems, navigation systems, and methods involving a robotic manipulator configured to control movement of an instrument to facilitate a surgical procedure. The navigation system includes a camera unit configured to optically track a pose of the instrument and a non-optical sensor coupled to the instrument. The navigation system includes a computing system coupled to the camera unit and being configured to obtain readings from the non-optical sensor. The computing system detects a condition whereby the camera unit is blocked from optically tracking the pose of the instrument. In response to detection of the condition, the computing system tracks the pose of the instrument with the readings from the non-optical sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/783,654, filed on Feb. 6, 2020, which is a continuation of U.S.patent application Ser. No. 15/599,935, filed on May 19, 2017, issued asU.S. Pat. No. 10,575,906, which is a continuation of U.S. patentapplication Ser. No. 14/994,236, filed on Jan. 13, 2016, issued as U.S.Pat. No. 9,687,307, which is a continuation of U.S. patent applicationSer. No. 14/635,402, filed on Mar. 2, 2015, issued as U.S. Pat. No.9,271,804, which is a continuation of U.S. patent application Ser. No.14/035,207, filed on Sep. 24, 2013, issued as U.S. Pat. No. 9,008,757,which claims priority to and the benefit of U.S. Provisional Patent App.No. 61/705,804, filed on Sep. 26, 2012, the entire contents of all ofwhich are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to navigation systems andmethods that tracks objects in space by determining changes in theposition and/or orientation of such objects over time. Morespecifically, the present disclosure relates to navigation systems andmethods that utilize optical sensors and non-optical sensors todetermine the position and/or orientation of objects.

BACKGROUND OF THE INVENTION

Navigation systems assist users in precisely locating objects. Forinstance, navigation systems are used in industrial, aerospace, defense,and medical applications. In the medical field, navigation systemsassist surgeons in precisely placing surgical instruments relative to apatient's anatomy.

Surgeries in which navigation systems are used include neurosurgery andorthopedic surgery. Often the instrument and the anatomy are trackedtogether with their relative movement shown on a display. The navigationsystem may display the instrument moving in conjunction with apreoperative image or an intraoperative image of the anatomy.Preoperative images are typically prepared by MRI or CT scans, whileintraoperative images may be prepared using a fluoroscope, low levelx-ray or any similar device. Alternatively, some systems are image-lessin which the patient's anatomy is “painted” by a navigation probe andmathematically fitted to an anatomical model for display.

Navigation systems may employ light signals, sound waves, magneticfields, RF signals, etc. in order to track the position and/ororientation of the instrument and anatomy. Optical navigation systemsare widely used due to the accuracy of such systems.

Prior art optical navigation systems typically include one or morecamera units that house one or more optical sensors (such as chargecoupled devices or CCDs). The optical sensors detect light emitted fromtrackers attached to the instrument and the anatomy. Each tracker has aplurality of optical emitters such as light emitting diodes (LEDs) thatperiodically transmit light to the sensors to determine the position ofthe LEDs.

The positions of the LEDs on the instrument tracker correlate to thecoordinates of a working end of the instrument relative to a cameracoordinate system. The positions of the LEDs on the anatomy tracker(s)correlate to the coordinates of a target area of the anatomy inthree-dimensional space relative to the camera coordinate system. Thus,the position and/or orientation of the working end of the instrumentrelative to the target area of the anatomy can be tracked and displayed.

Navigation systems can be used in a closed loop manner to controlmovement of surgical instruments. In these navigation systems both theinstrument and the anatomy being treated are outfitted with trackerssuch that the navigation system can track their position andorientation. Information from the navigation system is then fed to acontrol system to control or guide movement of the instrument. In somecases, the instrument is held by a robot and the information is sentfrom the navigation system to a control system of the robot.

In order for the control system to quickly account for relative motionbetween the instrument and the anatomy being treated, the accuracy andspeed of the navigation system must meet the desired tolerances of theprocedure. For instance, tolerances associated with cementless kneeimplants may be very small to ensure adequate fit and function of theimplant. Accordingly, the accuracy and speed of the navigation systemmay need to be greater than in more rough cutting procedures.

One of the limitations on accuracy and speed of optical navigationsystems is that the system relies on the line-of-sight between the LEDsand the optical sensors of the camera unit. When the line-of-sight isbroken, the system may not accurately determine the position and/ororientation of the instrument and anatomy being tracked. As a result,surgeries can encounter many starts and stops. For instance, duringcontrol of robotically assisted cutting, when the line-of-sight isbroken, the cutting tool must be disabled until the line-of-sight isregained. This can cause significant delays and added cost to theprocedure.

Another limitation on accuracy occurs when using active LEDs on thetrackers. In such systems, the LEDs are often fired in sequence. In thiscase only the position of the actively fired LED is measured and knownby the system, while the positions of the remaining, unmeasured LEDs areunknown. In these systems, the positions of the remaining, unmeasuredLEDs are approximated. Approximations are usually based on linearvelocity data extrapolated from the last known measured positions of thecurrently unmeasured LEDs. However, because the LEDs are fired insequence, there can be a considerable lag between measurements of anyone LED. This lag is increased with each additional tracker used in thesystem. Furthermore, this approximation does not take into accountrotations of the trackers, resulting in further possible errors inposition data for the trackers.

As a result, there is a need in the art for an optical navigation systemthat utilizes additional non-optically based data to improve trackingand provide a level of accuracy and speed with which to determineposition and/or orientations of objects for precise surgical proceduressuch as robotically assisted surgical cutting.

SUMMARY

In one example, a surgical system is provided comprising: a roboticmanipulator; an instrument coupled to the robotic manipulator, whereinthe robotic manipulator is configured to control movement of theinstrument to facilitate a surgical procedure; a non-optical sensorcoupled to the instrument; and a navigation system configured obtainreadings from the non-optical sensor, and the navigation systemcomprising a camera unit configured to optically track a pose of theinstrument, wherein the navigation system is configured to: detect acondition whereby the camera unit is blocked from optically tracking thepose of the instrument; and in response to detection of the condition,track the pose of the instrument with the readings from the non-opticalsensor.

In one example, a navigation system is provided that is configured to beutilized with a robotic manipulator configured to control movement of aninstrument to facilitate a surgical procedure, the navigation systemcomprising: a camera unit configured to optically track a pose of theinstrument; a non-optical sensor coupled to the instrument; and acomputing system coupled to the camera unit and being configured toobtain readings from the non-optical sensor, and the computing systembeing configured to: detect a condition whereby the camera unit isblocked from optically tracking the pose of the instrument; and inresponse to detection of the condition, track the pose of the instrumentwith the readings from the non-optical sensor.

In one example, a method is provided of operating a navigation systemincluding a camera unit, a non-optical sensor, and a computing system,the non-optical sensor being coupled to an instrument that is controlledby a robotic manipulator, the method comprising: optically tracking apose of the instrument with the camera unit; detecting, with thecomputing system, a condition whereby the camera unit is blocked fromoptically tracking the pose of the instrument; obtaining, with thecomputing system, readings from the non-optical sensor; and in responseto detecting the condition, the computing system tracking the pose ofthe instrument with the readings from the non-optical sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention will be readily appreciated as thesame becomes better understood by reference to the following detaileddescription when considered in connection with the accompanying drawingswherein:

FIG. 1 is a perspective view of a navigation system of the presentinvention being used in conjunction with a robotic manipulator;

FIG. 2 is a schematic view of the navigation system;

FIG. 3 is schematic view of coordinate systems used with the navigationsystem;

FIG. 4 is a flow diagram of steps carried out by a localization engineof the navigation system;

FIG. 4A is a schematic illustration of matching measured LEDs with atracker model to obtain a transformation matrix;

FIG. 5 is a flow diagram of steps carried out by the localization enginein a first alternative embodiment;

FIG. 5A is an illustration of a tracker model including real and virtualLEDs;

FIG. 6 is a flow diagram of steps carried out by the localization enginein a second alternative embodiment; and

FIG. 7 is a flow diagram of steps carried out by the localization enginewhen one or more LEDs are blocked from measurement.

DETAILED DESCRIPTION I. Overview

Referring to FIG. 1 a surgical navigation system 20 is illustrated.System 20 is shown in a surgical setting such as an operating room of amedical facility. The navigation system 20 is set up to track movementof various objects in the operating room. Such objects include, forexample, a surgical instrument 22, a femur F of a patient, and a tibia Tof the patient. The navigation system 20 tracks these objects forpurposes of displaying their relative positions and orientations to thesurgeon and, in some cases, for purposes of controlling or constrainingmovement of the surgical instrument 22 relative to a predefined path oranatomical boundary.

The surgical navigation system 20 includes a computer cart assembly 24that houses a navigation computer 26. A navigation interface is inoperative communication with the navigation computer 26. The navigationinterface includes a display 28 adapted to be situated outside of thesterile field and a display 29 adapted to be situated inside the sterilefield. The displays 28, 29 are adjustably mounted to the computer cartassembly 24. Input devices 30, 32 such as a mouse and keyboard can beused to input information into the navigation computer 26 or otherwiseselect/control certain aspects of the navigation computer 26. Otherinput devices are contemplated including a touch screen (not shown) ondisplays 28, 29 or voice-activation.

A localizer 34 communicates with the navigation computer 26. In theembodiment shown, the localizer 34 is an optical localizer and includesa camera unit 36. The camera unit 36 has an outer casing 38 that housesone or more optical sensors 40. In some embodiments at least two opticalsensors 40 are employed, preferably three. The optical sensors 40 may bethree separate high resolution charge-coupled devices (CCD). In oneembodiment three, one-dimensional CCDs are employed. It should beappreciated that in other embodiments, separate camera units, each witha separate CCD, or two or more CCDs, could also be arranged around theoperating room. The CCDs detect infrared (IR) signals.

Camera unit 36 is mounted on an adjustable arm to position the opticalsensors 40 above the zone in which the procedure is to take place toprovide the camera unit 36 with a field of view of the below discussedtrackers that, ideally, is free from obstructions.

The camera unit 36 includes a camera controller 42 in communication withthe optical sensors 40 to receive signals from the optical sensors 40.The camera controller 42 communicates with the navigation computer 26through either a wired or wireless connection (not shown). One suchconnection may be an IEEE 1394 interface, which is a serial businterface standard for high-speed communications and isochronousreal-time data transfer. Connection could also use a company specificprotocol. In other embodiments, the optical sensors 40 communicatedirectly with the navigation computer 26.

Position and orientation signals and/or data are transmitted to thenavigation computer 26 for purposes of tracking the objects. Thecomputer cart assembly 24, display 28, and camera unit 36 may be likethose described in U.S. Pat. No. 7,725,162 to Malackowski, et al. issuedon May 25, 2010, entitled “Surgery System”, hereby incorporated byreference.

The navigation computer 26 can be a personal computer or laptopcomputer. Navigation computer 26 has the display 28, central processingunit (CPU) and/or other processors, memory (not shown), and storage (notshown). The navigation computer 26 is loaded with software as describedbelow. The software converts the signals received from the camera unit36 into data representative of the position and orientation of theobjects being tracked.

Navigation system 20 includes a plurality of tracking devices 44, 46,48, also referred to herein as trackers. In the illustrated embodiment,one tracker 44 is firmly affixed to the femur F of the patient andanother tracker 46 is firmly affixed to the tibia T of the patient.Trackers 44, 46 are firmly affixed to sections of bone. Trackers 44, 46may be attached to the femur F in the manner shown in U.S. Pat. No.7,725,162, hereby incorporated by reference. In further embodiments, anadditional tracker (not shown) is attached to the patella to track aposition and orientation of the patella. In further embodiments, thetrackers 44, 46 could be mounted to other tissue types or parts of theanatomy.

An instrument tracker 48 is firmly attached to the surgical instrument22. The instrument tracker 48 may be integrated into the surgicalinstrument 22 during manufacture or may be separately mounted to thesurgical instrument 22 in preparation for the surgical procedures. Theworking end of the surgical instrument 22, which is being tracked, maybe a rotating bur, electrical ablation device, or the like. In theembodiment shown, the surgical instrument 22 is an end effector of asurgical manipulator. Such an arrangement is shown in U.S. ProvisionalPatent Application No. 61/679,258, entitled, “Surgical ManipulatorCapable of Controlling a Surgical Instrument in either a Semi-AutonomousMode or a Manual, Boundary Constrained Mode”, the disclosure of which ishereby incorporated by reference, and also in U.S. patent applicationSer. No. 13/958,834, entitled, “Navigation System for use with aSurgical Manipulator Operable in Manual or Semi-Autonomous Mode”, thedisclosure of which is hereby incorporated by reference.

The trackers 44, 46, 48 can be battery powered with an internal batteryor may have leads to receive power through the navigation computer 26,which, like the camera unit 36, preferably receives external power.

In other embodiments, the surgical instrument 22 may be manuallypositioned by only the hand of the user, without the aid of any cuttingguide, jib, or other constraining mechanism such as a manipulator orrobot. Such a surgical instrument is described in U.S. ProvisionalPatent Application No. 61/662,070, entitled, “Surgical InstrumentIncluding Housing, a Cutting Accessory that Extends from the Housing andActuators that Establish the Position of the Cutting Accessory Relativeto the Housing”, hereby incorporated by reference, and also in U.S.patent application Ser. No. 13/600,888, entitled “Surgical InstrumentIncluding Housing, a Cutting Accessory that Extends from the Housing andActuators that Establish the Position of the Cutting Accessory Relativeto the Housing”, hereby incorporated by reference.

The optical sensors 40 of the localizer 34 receive light signals fromthe trackers 44, 46, 48. In the illustrated embodiment, the trackers 44,46, 48 are active trackers. In this embodiment, each tracker 44, 46, 48has at least three active markers 50 for transmitting light signals tothe optical sensors 40. The active markers 50 can be light emittingdiodes or LEDs 50. The optical sensors 40 preferably have sampling ratesof 100 Hz or more, more preferably 300 Hz or more, and most preferably500 Hz or more. In some embodiments, the optical sensors 40 havesampling rates of 1000 Hz. The sampling rate is the rate at which theoptical sensors 40 receive light signals from sequentially fired LEDs50. In some embodiments, the light signals from the LEDs 50 are fired atdifferent rates for each tracker 44, 46, 48.

Referring to FIG. 2 , each of the LEDs 50 are connected to a trackercontroller 62 located in a housing (not shown) of the associated tracker44, 46, 48 that transmits/receives data to/from the navigation computer26. In one embodiment, the tracker controllers 62 transmit data on theorder of several Megabytes/second through wired connections with thenavigation computer 26. In other embodiments, a wireless connection maybe used. In these embodiments, the navigation computer 26 has atransceiver (not shown) to receive the data from the tracker controller62.

In other embodiments, the trackers 44, 46, 48 may have passive markers(not shown), such as reflectors that reflect light emitted from thecamera unit 36. The reflected light is then received by the opticalsensors 40. Active and passive arrangements are well known in the art.

Each of the trackers 44, 46, 48 also includes a 3-dimensional gyroscopesensor 60 that measures angular velocities of the trackers 44, 46, 48.As is well known to those skilled in the art, the gyroscope sensors 60output readings indicative of the angular velocities relative to x-, y-,and z-axes of a gyroscope coordinate system. These readings aremultiplied by a conversion constant defined by the manufacturer toobtain measurements in degrees/second with respect to each of the x-,y-, and z-axes of the gyroscope coordinate system. These measurementscan then be converted to an angular velocity vector {right arrow over(ω)} defined in radians/second.

The angular velocities measured by the gyroscope sensors 60 provideadditional non-optically based kinematic data for the navigation system20 with which to track the trackers 44, 46, 48. The gyroscope sensors 60may be oriented along the axis of each coordinate system of the trackers44, 46, 48. In other embodiments, each gyroscope coordinate system istransformed to its tracker coordinate system such that the gyroscopedata reflects the angular velocities with respect to the x-, y-, andz-axes of the coordinate systems of the trackers 44, 46, 48.

Each of the gyroscope sensors 60 communicate with the tracker controller62 located within the housing of the associated tracker thattransmits/receives data to/from the navigation computer 26. Thenavigation computer 26 has one or more transceivers (not shown) toreceive the data from the gyroscope sensors 60. The data can be receivedeither through a wired or wireless connection.

The gyroscope sensors 60 preferably have sampling rates of 100 Hz ormore, more preferably 300 Hz or more, and most preferably 500 Hz ormore. In some embodiments, the gyroscope sensors 60 have sampling ratesof 1000 Hz. The sampling rate of the gyroscope sensors 60 is the rate atwhich signals are sent out from the gyroscope sensors 60 to be convertedinto angular velocity data.

The sampling rates of the gyroscope sensors 60 and the optical sensors40 are established or timed so that for each optical measurement ofposition there is a corresponding non-optical measurement of angularvelocity.

Each of the trackers 44, 46, 48 also includes a 3-axis accelerometer 70that measures acceleration along each of x-, y-, and z-axes of anaccelerometer coordinate system. The accelerometers 70 provideadditional non-optically based data for the navigation system 20 withwhich to track the trackers 44, 46, 48.

Each of the accelerometers 70 communicate with the tracker controller 62located in the housing of the associated tracker that transmits/receivesdata to/from the navigation computer 26. One or more of the transceivers(not shown) of the navigation computer receives the data from theaccelerometers 70.

The accelerometers 70 may be oriented along the axis of each coordinatesystem of the trackers 44, 46, 48. In other embodiments, eachaccelerometer coordinate system is transformed to its tracker coordinatesystem such that the accelerometer data reflects the accelerations withrespect to the x-, y-, and z-axes of the coordinate systems of thetrackers 44, 46, 48.

The navigation computer 26 includes a navigation processor 52. Thecamera unit 36 receives optical signals from the LEDs 50 of the trackers44, 46, 48 and outputs to the processor 52 signals relating to theposition of the LEDs 50 of the trackers 44, 46, 48 relative to thelocalizer 34. The gyroscope sensors 60 transmit non-optical signals tothe processor 52 relating to the 3-dimensional angular velocitiesmeasured by the gyroscope sensors 60. Based on the received optical andnon-optical signals, navigation processor 52 generates data indicatingthe relative positions and orientations of the trackers 44, 46, 48relative to the localizer 34.

It should be understood that the navigation processor 52 could includeone or more processors to control operation of the navigation computer26. The processors can be any type of microprocessor or multi-processorsystem. The term processor is not intended to limit the scope of theinvention to a single processor.

Prior to the start of the surgical procedure, additional data are loadedinto the navigation processor 52. Based on the position and orientationof the trackers 44, 46, 48 and the previously loaded data, navigationprocessor 52 determines the position of the working end of the surgicalinstrument 22 and the orientation of the surgical instrument 22 relativeto the tissue against which the working end is to be applied. In someembodiments, navigation processor 52 forwards these data to amanipulator controller 54. The manipulator controller 54 can then usethe data to control a robotic manipulator 56 as described in U.S.Provisional Patent Application No. 61/679,258, entitled, “SurgicalManipulator Capable of Controlling a Surgical Instrument in either aSemi-Autonomous Mode or a Manual, Boundary Constrained Mode, thedisclosure of which is hereby incorporated by reference, and also inU.S. patent application Ser. No. 13/958,834, entitled, “NavigationSystem for use with a Surgical Manipulator Operable in Manual orSemi-Autonomous Mode”, the disclosure of which is hereby incorporated byreference.

The navigation processor 52 also generates image signals that indicatethe relative position of the surgical instrument working end to thesurgical site. These image signals are applied to the displays 28, 29.Displays 28, 29, based on these signals, generate images that allow thesurgeon and staff to view the relative position of the surgicalinstrument working end to the surgical site. The displays, 28, 29, asdiscussed above, may include a touch screen or other input/output devicethat allows entry of commands.

II. Coordinate Systems and Transformation

Referring to FIG. 3 , tracking of objects is generally conducted withreference to a localizer coordinate system LCLZ. The localizercoordinate system has an origin and an orientation (a set of x-, y-, andz-axes). During the procedure one goal is to keep the localizercoordinate system LCLZ stationary. As will be described further below,an accelerometer mounted to the camera unit 36 may be used to tracksudden or unexpected movement of the localizer coordinate system LCLZ,as may occur when the camera unit 36 is inadvertently bumped by surgicalpersonnel.

Each tracker 44, 46, 48 and object being tracked also has its owncoordinate system separate from localizer coordinate system LCLZ.Components of the navigation system 20 that have their own coordinatesystems are the bone trackers 44, 46 and the instrument tracker 48.These coordinate systems are represented as, respectively, bone trackercoordinate systems BTRK1, BTRK2, and instrument tracker coordinatesystem TLTR.

Navigation system 20 monitors the positions of the femur F and tibia Tof the patient by monitoring the position of bone trackers 44, 46 firmlyattached to bone. Femur coordinate system is FBONE and tibia coordinatesystem is TBONE, which are the coordinate systems of the bones to whichthe bone trackers 44, 46 are firmly attached.

Prior to the start of the procedure, pre-operative images of the femur Fand tibia T are generated (or of other tissues in other embodiments).These images may be based on MRI scans, radiological scans or computedtomography (CT) scans of the patient's anatomy. These images are mappedto the femur coordinate system FBONE and tibia coordinate system TBONEusing well known methods in the art. In one embodiment, a pointerinstrument P, such as disclosed in U.S. Pat. No. 7,725,162 toMalackowski, et al., hereby incorporated by reference, having its owntracker PT (see FIG. 2 ), may be used to map the femur coordinate systemFBONE and tibia coordinate system TBONE to the pre-operative images.These images are fixed in the femur coordinate system FBONE and tibiacoordinate system TBONE.

During the initial phase of the procedure, the bone trackers 44, 46 arefirmly affixed to the bones of the patient. The pose (position andorientation) of coordinate systems FBONE and TBONE are mapped tocoordinate systems BTRK1 and BTRK2, respectively. Given the fixedrelationship between the bones and their bone trackers 44, 46, the poseof coordinate systems FBONE and TBONE remain fixed relative tocoordinate systems BTRK1 and BTRK2, respectively, throughout theprocedure. The pose-describing data are stored in memory integral withboth manipulator controller 54 and navigation processor 52.

The working end of the surgical instrument 22 (also referred to asenergy applicator distal end) has its own coordinate system EAPP. Theorigin of the coordinate system EAPP may represent a centroid of asurgical cutting bur, for example. The pose of coordinate system EAPP isfixed to the pose of instrument tracker coordinate system TLTR beforethe procedure begins. Accordingly, the poses of these coordinate systemsEAPP, TLTR relative to each other are determined. The pose-describingdata are stored in memory integral with both manipulator controller 54and navigation processor 52.

III. Software

Referring to FIG. 2 , a localization engine 100 is a software modulethat can be considered part of the navigation system 20. Components ofthe localization engine 100 run on navigation processor 52. In someversions of the invention, the localization engine 100 may run on themanipulator controller 54.

Localization engine 100 receives as inputs the optically-based signalsfrom the camera controller 42 and the non-optically based signals fromthe tracker controller 62. Based on these signals, localization engine100 determines the pose (position and orientation) of the bone trackercoordinate systems BTRK1 and BTRK2 in the localizer coordinate systemLCLZ. Based on the same signals received for the instrument tracker 48,the localization engine 100 determines the pose of the instrumenttracker coordinate system TLTR in the localizer coordinate system LCLZ.

The localization engine 100 forwards the signals representative of theposes of trackers 44, 46, 48 to a coordinate transformer 102. Coordinatetransformer 102 is a navigation system software module that runs onnavigation processor 52. Coordinate transformer 102 references the datathat defines the relationship between the pre-operative images of thepatient and the patient trackers 44, 46. Coordinate transformer 102 alsostores the data indicating the pose of the working end of the surgicalinstrument relative to the instrument tracker 48.

During the procedure, the coordinate transformer 102 receives the dataindicating the relative poses of the trackers 44, 46, 48 to thelocalizer 34. Based on these data and the previously loaded data, thecoordinate transformer 102 generates data indicating the relativeposition and orientation of both the coordinate system EAPP, and thebone coordinate systems, FBONE and TBONE to the localizer coordinatesystem LCLZ.

As a result, coordinate transformer 102 generates data indicating theposition and orientation of the working end of the surgical instrument22 relative to the tissue (e.g., bone) against which the instrumentworking end is applied. Image signals representative of these data areforwarded to displays 28, 29 enabling the surgeon and staff to view thisinformation. In certain embodiments, other signals representative ofthese data can be forwarded to the manipulator controller 54 to controlthe manipulator 56 and corresponding movement of the surgical instrument22.

Steps for determining the pose of each of the tracker coordinate systemsBTRK1, BTRK2, TLTR in the localizer coordinate system LCLZ are the same,so only one will be described in detail. The steps shown in FIG. 4 arebased on only one tracker being active, tracker 44. In the followingdescription, the LEDs of tracker 44 shall be represented by numerals 50a, 50 b, 50 c which identify first 50 a, second 50 b, and third 50 cLEDs.

The steps set forth in FIG. 4 illustrate the use of optically-basedsensor data and non-optically based sensor data to determine thepositions of the LEDs 50 a, 50 b, 50 c of tracker 44. From thesepositions, the navigation processor 52 can determine the position andorientation of the tracker 44, and thus, the position and orientation ofthe femur F to which it is attached. Optically-based sensor data derivedfrom the signals received by the optical sensors 40 provideline-of-sight based data that relies on the line-of-sight between theLEDs 50 a, 50 b, 50 c and the optical sensors 40. However, the gyroscopesensor 60, which provides non-optically based signals for generatingnon-optically based sensor data do not rely on line-of-sight and thuscan be integrated into the navigation system 20 to better approximatepositions of the LEDs 50 a, 50 b, 50 c when two of the LEDs 50 a, 50 b,50 c are not being measured (since only one LED measured at a time), orwhen one or more of the LEDs 50 a, 50 b, 50 c are not visible to theoptical sensors 40 during a procedure.

In a first initialization step 200, the system 20 measures the positionof the LEDs 50 a, 50 b, 50 c for the tracker 44 in the localizercoordinate system LCLZ to establish initial position data. Thesemeasurements are taken by sequentially firing the LEDs 50 a, 50 b, 50 c,which transmits light signals to the optical sensors 40. Once the lightsignals are received by the optical sensors 40, corresponding signalsare generated by the optical sensors 40 and transmitted to the cameracontroller 42. The frequency between firings of the LEDs 50 a, 50 b, 50c is 100 Hz or greater, preferably 300 Hz or greater, and morepreferably 500 Hz or greater. In some cases, the frequency betweenfirings is 1000 Hz or 1 millisecond between firings.

In some embodiments, only one LED can be read by the optical sensors 40at a time. The camera controller 42, through one or more infrared or RFtransceivers (on camera unit 36 and tracker 44) may control the firingof the LEDs 50 a, 50 b, 50 c, as described in U.S. Pat. No. 7,725,162 toMalackowski, et al., hereby incorporated by reference. Alternatively,the tracker 44 may be activated locally (such as by a switch on tracker44) which then fires its LEDs 50 a, 50 b, 50 c sequentially onceactivated, without instruction from the camera controller 42.

Based on the inputs from the optical sensors 40, the camera controller42 generates raw position signals that are then sent to the localizationengine 100 to determine the position of each of the corresponding threeLEDs 50 a, 50 b, 50 c in the localizer coordinate system LCLZ.

During the initialization step 200, in order to establish the initialposition data, movement of the tracker 44 must be less than apredetermined threshold. A value of the predetermined threshold isstored in the navigation computer 26. The initial position dataestablished in step 200 essentially provides a static snapshot ofposition of the three LEDs 50 a, 50 b, 50 c at an initial time t0, fromwhich to base the remaining steps of the process. During initialization,velocities of the LEDs 50 a, 50 b, 50 c are calculated by thelocalization engine 100 between cycles (i.e., each set of three LEDmeasurements) and once the velocities are low enough, i.e., less thanthe predetermined threshold showing little movement occurred, then theinitial position data or static snapshot is established. In someembodiments, the predetermined threshold (also referred to as the staticvelocity limit) is 200 mm/s or less, preferably 100 mm/s or less, andmore preferably 10 mm/s or less along any axis. When the predeterminedthreshold is 100 mm/s, then the calculated velocities must be less than100 mm/s to establish the static snapshot.

Referring to FIGS. 4 and 4A, once the static snapshot is taken, thepositions of the measured LEDs 50 a, 50 b, 50 c are compared to a modelof the tracker 44 in step 202. The model is data stored in thenavigation computer 26. The model data indicates the positions of theLEDs on the tracker 44 in the tracker coordinate system BTRK1. Thesystem 20 has stored the number and position of the LEDs 50 of eachtracker 44, 46, 48 in each tracker's coordinate system. For trackers 44,46, 48 the origin of their coordinate systems is set at the centroid ofall LED positions of the tracker 44.

The localization engine 100 utilizes a rigid body matching algorithm orpoint matching algorithm to match the measured LEDs 50 a, 50 b, 50 c inthe localizer coordinate system LCLZ to the LEDs in the stored model.Once the best-fit is determined, the localization engine 100 evaluatesthe deviation of the fit to determine if the measured LEDs 50 a, 50 b,50 c fit within a stored predefined tolerance of the model. Thetolerance may be based on a distance between the corresponding LEDs suchthat if the fit results in too great of a distance, the initializationstep has to be repeated. In some embodiments, the positions of the LEDsmust not deviate from the model by more than 2.0 mm, preferably not morethan 0.5 mm, and more preferably not more than 0.1 mm.

If the fit is within the predefined tolerance, a transformation matrixis generated to transform any other unmeasured LEDs in the model fromthe bone tracker coordinate system BTRK1 into the localizer coordinatesystem LCLZ in step 204. This step is utilized if more than three LEDsare used or if virtual LEDs are used as explained further below. In someembodiments, trackers 44, 46, 48 may have four or more LEDs. Once allpositions in the localizer coordinate system LCLZ are established, anLED cloud is created. The LED cloud is an arrangement of all LEDs 50 a,50 b, 50 c on the tracker 44 in the localizer coordinate system LCLZbased on the x-, y-, and z-axis positions of all the LEDs 50 a, 50 b, 50c in the localizer coordinate system LCLZ.

Once the LED cloud is initially established, the navigation system 20can proceed with tracking the tracker 44 during a surgical procedure. Aspreviously discussed, this includes firing the next LED in the sequence.For illustration, LED 50 a is now fired. Thus, LED 50 a transmits lightsignals to the optical sensors 40. Once the light signals are receivedby the optical sensors 40, corresponding signals are generated by theoptical sensors 40 and transmitted to the camera controller 42.

Based on the inputs from the optical sensors 40, the camera controller42 generates a raw position signal that is then sent to the localizationengine 100 to determine at time t1 the new position of LED 50 a relativeto the x-, y-, and z-axes of the localizer coordinate system LCLZ. Thisis shown in step 206 as a new LED measurement.

It should be appreciated that the designation of time such as t0, t1 . .. tn is used for illustrative purposes to indicate different times ordifferent ranges of time or time periods and does not limit thisinvention to specific or definitive times.

With the new position of LED 50 a determined, a linear velocity vectorof LED 50 a can be calculated by the localization engine 100 in step208.

The tracker 44 is treated as a rigid body. Accordingly, the linearvelocity vector of LED 50 a is a vector quantity, equal to the time rateof change of its linear position. The velocity, even the acceleration ofeach LED, in localizer coordinate system LCLZ, can be calculated fromthe previously and currently measured positions and time of that LED inlocalizer coordinate system LCLZ. The previously and currently measuredpositions and time of a LED define the position history of that LED. Thevelocity calculation of LED 50 a can take the simplest form of:

${\overset{\rightarrow}{v}\left( {{LED}50a} \right)} = \frac{{\overset{\rightarrow}{x}}_{n} - {\overset{\rightarrow}{x}}_{p}}{t_{n} - t_{p}}$

Where {right arrow over (x)}_(p)=(x, y, z)_(p) and is the previouslymeasured position of LED 50 a at time t_(p); and {right arrow over(x)}_(n)=(x, y, z)_(n) and is the currently measured position of LED 50a at time t_(n). One can also obtain the velocity and/or acceleration ofeach LED by data fitting the LED position history of that LED as is wellknown to those skilled in the art.

At time t1, in step 210, the gyroscope sensor 60 is also measuring anangular velocity of the tracker 44. Gyroscope sensor 60 transmitssignals to the tracker controller 62 related to this angular velocity.

The tracker controller 62 then transmits a corresponding signal to thelocalization engine 100 so that the localization engine 100 cancalculate an angular velocity vector {right arrow over (ω)} from thesesignals. In step 210, the gyroscope coordinate system is alsotransformed to the bone tracker coordinate system BTRK1 so that theangular velocity vector {right arrow over (ω)} calculated by thelocalization engine 100 is expressed in the bone tracker coordinatesystem BTRK1.

In step 212, a relative velocity vector {right arrow over (v)} iscalculated for the origin of the bone tracker coordinate system BTRK1with respect to position vector {right arrow over (x)} (LED50 a toORIGIN). This position vector {right arrow over (x)} (LED50 a to ORIGIN)is also stored in memory in the navigation computer 26 for access by thelocalization engine 100 for the following calculation. This calculationdetermines the relative velocity of the origin {right arrow over (v)}R(ORIGIN) of the bone tracker coordinate system BTRK1 by calculating thecross product of the angular velocity vector {right arrow over (ω)}derived from the gyroscope signal and the position vector from LED 50 ato the origin.

{right arrow over (v)}R(ORIGIN)={right arrow over (ω)}×{right arrow over(x)}(LED50a to ORIGIN)

The localization engine 100 then calculates relative velocity vectors{right arrow over (v)}R for the remaining, unmeasured LEDs 50 b, 50 c(unmeasured because these LEDs have not been fired and thus theirpositions are not being measured). These velocity vectors can becalculated with respect to the origin of bone tracker coordinate systemBTRK1.

The calculation performed by the localization engine 100 to determinethe relative velocity vector {right arrow over (v)}R for each unmeasuredLED 50 b, 50 c at time t1 is based on the cross product of the angularvelocity vector {right arrow over (ω)} at time t1 and the positionvectors {right arrow over (x)} (ORIGIN to LED50 b) and {right arrow over(x)} (ORIGIN to LED50 c), which are taken from the origin of bonetracker coordinate system BTRK1 to each of the unmeasured LEDs 50 b, 50c. These position vectors {right arrow over (x)} (ORIGIN to LED 50 b)and {right arrow over (x)} (ORIGIN to LED50 c) are stored in memory inthe navigation computer 26 for access by the localization engine 100 forthe following calculations:

{right arrow over (v)}R(LED50b)={right arrow over (ω)}×{right arrow over(x)}(ORIGIN to LED50b)

{right arrow over (v)}R(LED50c)={right arrow over (ω)}×{right arrow over(x)}(ORIGIN to LED50c)

Also in step 212, these relative velocities, which are calculated in thebone tracker coordinate system BTRK1, are transferred into the localizercoordinate system LCLZ using the transformation matrix determined instep 202. The relative velocities in the localizer coordinate systemLCLZ are used in calculations in step 214.

In step 214 the velocity vector {right arrow over (v)} of the origin ofthe bone tracker coordinate system BTRK1 in the localizer coordinatesystem LCLZ at time t1 is first calculated by the localization engine100 based on the measured velocity vector {right arrow over (v)} (LED50a) of LED 50 a at time t1. The velocity vector (ORIGIN) is calculated byadding the velocity vector {right arrow over (v)} (LED50 a) of LED 50 aat time t1 and the relative velocity vector {right arrow over (v)}R(ORIGIN) of the origin at time t1 expressed relative to the positionvector of LED 50 a to the origin. Thus, the velocity vector of theorigin at time t1 is calculated as follows:

{right arrow over (v)}(ORIGIN)={right arrow over (v)}(LED50a)+{rightarrow over (v)}R(ORIGIN)

Velocity vectors of the remaining, unmeasured LEDs in the localizercoordinate system LCLZ at time t1 can now be calculated by thelocalization engine 100 based on the velocity vector {right arrow over(v)} (ORIGIN) of the origin of the bone tracker coordinate system BTRK1in the localizer coordinate system LCLZ at time t1 and their respectiverelative velocity vectors at time t1 expressed relative to theirposition vectors with the origin of the bone tracker coordinate systemBTRK1. These velocity vectors at time t1 are calculated as follows:

{right arrow over (v)}(LED50b)={right arrow over (v)}(ORIGIN)+{rightarrow over (v)}R(LED50b)

{right arrow over (v)}(LED50c)={right arrow over (v)}(ORIGIN)+{rightarrow over (v)}R(LED50c)

In step 216, the localization engine 100 calculates the movements, i.e.,the change in position Δx (in Cartesian coordinates), of each of theunmeasured LEDs 50 b, 50 c from time t0 to time t1 based on thecalculated velocity vectors of LEDs 50 b, 50 c and the change in time.In some embodiments the change in time Δt for each LED measurement istwo milliseconds or less, and in some embodiments one millisecond orless.

Δx(LED50b)={right arrow over (v)}(LED50b)×Δt

Δx(LED50c)={right arrow over (v)}(LED50c)×Δt

These calculated changes in position (x, y, z) can then be added to thepreviously determined positions of each of LEDs 50 b, 50 c in thelocalizer coordinate system LCLZ. Thus, in step 218, changes in positioncan be added to the previous positions of the LEDs 50 b, 50 c at timet0, which were determined during the static snapshot. This is expressedas follows:

x(LED50b)_(t1) =x(LED50b)_(t0) +Δx(LED50b)

In step 220, these calculated positions for each of LEDs 50 b, 50 c attime t1 are combined with the determined position of LED 50 a at timet1. The newly determined positions of LEDs 50 a, 50 b, 50 c are thenmatched to the model of tracker 44 to obtain a best fit using the pointmatching algorithm or rigid body matching algorithm. The result of thisbest fit calculation, if within the defined tolerance of the system 20,is that a new transformation matrix is created by the navigationprocessor 52 to link the bone tracker coordinate system BTRK1 to thelocalizer coordinate system LCLZ.

With the new transformation matrix the newly calculated positions of theunmeasured LEDs 50 b, 50 c are adjusted to the model in step 222 toprovide adjusted positions. The measured position of LED 50 a can alsobe adjusted due to the matching algorithm such that it is alsorecalculated. These adjustments are considered an update to the LEDcloud. In some embodiments, the measured position of LED 50 a is fixedto the model's position of LED 50 a during the matching step.

With the best fit transformation complete, the measured (and possiblyadjusted) position of LED 50 a and the calculated (and adjusted)positions of LEDs 50 b, 50 c in the localizer coordinate system LCLZenable the coordinate transformer 102 to determine a new position andorientation of the femur F based on the previously describedrelationships between the femur coordinate system FBONE, the bonetracker coordinate system BTRK1, and the localizer coordinate systemLCLZ.

Steps 206 through 222 are then repeated at a next time t2 and start withthe measurement in the localizer coordinate system LCLZ of LED 50 b,with LEDs 50 a, 50 c being the unmeasured LEDs. As a result of this loopat each time t1, t2 . . . tn positions of each LED 50 a, 50 b, 50 c areeither measured (one LED being fired at each time) or calculated withthe calculated positions being very accurately approximated based onmeasurements by the optical sensor 40 and the gyroscope sensor 60. Thisloop of steps 206 through 222 to determine the new positions of the LEDs50 can be carried out by the localization engine 100 at a frequency ofat least 100 Hz, more preferably at least 300 Hz, and most preferably atleast 500 Hz.

Referring to FIG. 5 , the LED cloud may also include virtual LEDs, whichare predetermined points identified on the model, but that do notactually correspond to physical LEDs on the tracker 44. The positions ofthese points may also be calculated at times t1, t2 . . . tn. Thesevirtual LEDs can be calculated in the same fashion as the unmeasuredLEDs with reference to FIG. 4 . The only difference is that the virtualLEDs are never fired or included in the sequence of opticalmeasurements, since they do not correspond to any light source, but aremerely virtual in nature. Steps 300-322 show the steps used for trackingthe trackers 44, 46, 48 using real and virtual LEDS. Steps 300-322generally correspond to steps 200-222 except for the addition of thevirtual LEDs, which are treated like unmeasured LEDs using the sameequations described above.

One purpose of using virtual LEDs in addition to the LEDs 50 a, 50 b, 50c, for example, is to reduce the effect of errors in the velocitycalculations described above. These errors may have little consequenceon the calculated positions of the LEDs 50 a, 50 b, 50 c, but can beamplified the further away from the LEDs 50 a, 50 b, 50 c a point ofinterest is located. For instance, when tracking the femur F withtracker 44, the LEDs 50 a, 50 b, 50 c incorporated in the tracker 44 mayexperience slight errors in their calculated positions of about 0.2millimeters. However, consider the surface of the femur F that may belocated over 10 centimeters away from the LEDs 50 a, 50 b, 50 c. Theslight error of 0.2 millimeters at the LEDs 50 a, 50 b, 50 c can resultin 0.4 to 2 millimeters of error on the surface of the femur F. Thefurther away the femur F is located from the LEDs 50 a, 50 b, 50 c themore the error increases. The use of virtual LEDs in the steps of FIG. 5can reduce the potential amplification of such errors as describedbelow.

Referring to FIG. 5A, one virtual LED 50 d can be positioned on thesurface of the femur F. Other virtual LEDs 50 e, 50 f, 50 g, 50 h, 50 i,50 j, 50 k can be positioned at random locations in the bone trackercoordinate system BTRK1 such as along each of the x-, y-, and z-axes,and on both sides of the origin along these axes to yield 6 virtualLEDs. These virtual LEDs are included as part of the model of thetracker 44 shown in FIG. 5A and used in steps 302 and 320. In someembodiments, only the virtual LEDs 50 e-50 k are used. In otherembodiments, virtual LEDs may be positioned at locations along each ofthe x-, y-, and z-axes, but at different distances from the origin ofthe bone tracker coordinate system BTRK1. In still further embodiments,some or all of the virtual LEDs may be located off of the axes x-, y-,and z-.

Now in the model are real LEDs 50 a, 50 b, 50 c and virtual LEDs 50 d-50k. At each time t1, t2 . . . tn this extended model is matched in step320 with the measured/calculated positions of real LEDs 50 a, 50 b, 50 cand with the calculated positions of virtual LEDs 50 d-50 k to obtainthe transformation matrix that links the bone tracker coordinate systemBTRK1 with the localizer coordinate system LCLZ. Now, with the virtualLEDs 50 d-50 k included in the model, which are located at positionsoutlying the real LEDs 50 a, 50 b, 50 c, the error in the rotationmatrix can be reduced. In essence, the rigid body matching algorithm orpoint matching algorithm has additional points used for matching andsome of these additional points are located radially outwardly from thepoints defining the real LEDs 50 a, 50 b, 50 c, thus rotationallystabilizing the match.

In another variation of the process of FIG. 5 , the locations of thevirtual LEDs 50 e-50 k can be changed dynamically during use dependingon movement of the tracker 44. The calculated positions of theunmeasured real LEDs 50 b, 50 c and the virtual LEDs 50 e-50 k at timet1 are more accurate the slower the tracker 44 moves. Thus, thelocations of virtual LEDs 50 e-50 k along the x-, y-, and z-axesrelative to the origin of the bone tracker coordinate system BTRK1 canbe adjusted based on speed of the tracker 44. Thus, if the locations ofthe virtual LEDs 50 e-50 k are denoted (s,0,0), (-s,0,0), (0,s,0),(0,-s,0), (0,0,s), (0,0,-s), respectively, then s would increase whenthe tracker 44 moves slowly and s would decrease to a smaller value whenthe tracker 44 moves faster. This could be handled by an empiricalformula for s or s can be adjusted based on an estimate in the error invelocity and calculated positions.

Determining new positions of the LEDs 50 (real and virtual) can becarried out at a frequency of at least 100 Hz, more preferably at least300 Hz, and most preferably at least 500 Hz.

Data from the accelerometers 70 can be used in situations where opticalmeasurement of an LED 50 is impeded due to interference with theline-of-sight. When an LED to be measured is blocked, the localizationengine 100 assumes a constant velocity of the origin to estimatepositions. However, the constant velocity assumption in this situationmay be inaccurate and result in errors. The accelerometers 70essentially monitor if the constant velocity assumption in the timeperiod is accurate. The steps shown in FIGS. 6 and 7 illustrate how thisassumption is checked.

Continuing to use tracker 44 as an example, steps 400-422 of FIG. 6generally correspond to steps 300-322 from FIG. 5 . However, in step424, the system 20 determines whether, in the last cycle ofmeasurements, less than 3 LEDs were measured—meaning that one or more ofthe LEDs in the cycle could not be measured. This could be caused byline-of-sight issues, etc. A cycle for tracker 44 is the last threeattempted measurements. If during the last three measurements, each ofthe LEDs 50 a, 50 b, 50 c were visible and could be measured, then thesystem 20 proceeds to step 408 and continues as previously describedwith respect to FIG. 5 .

If the system 20 determines that one or more of the LEDs 50 a, 50 b, 50c could not be measured during the cycle, i.e., were blocked frommeasurement, then the algorithm still moves to step 408, but if the newLED to be measured in step 406 was the one that could not be measured,the system makes some velocity assumptions as described below.

When a LED, such as LED 50 a, is not seen by the optical sensor 40 atits measurement time to in step 406, the previously calculated velocityvector {right arrow over (v)} (ORIGIN) of the origin of the tracker 44in the localizer coordinate system LCLZ at the previous time t(n-1) isassumed to remain constant. Accordingly, velocity vectors of LEDs 50 a,50 b, 50 c in the localizer coordinate system LCLZ can be calculatedbased on the previously calculated velocity vector {right arrow over(v)} (ORIGIN) in the localizer coordinate system LCLZ and the relativevelocity vectors of LEDs 50 a, 50 b, 50 c, which are derived from anewly measured angular velocity vector from the gyroscope 60. Theequations described in steps 316-322 can then be used to determine newpositions of the LEDs 50 a, 50 b, 50 c.

To start, when LED 50 a is to be measured at step 406, but isobstructed, the velocity vector of the origin is assumed to be the sameas the previous calculation. Accordingly, the velocity of the new LED isnot calculated at step 408:

{right arrow over (v)}(ORIGIN)=previous calculation

Step 410 proceeds the same as step 310.

The relative velocity vectors {right arrow over (v)}R of LEDs 50 a, 50b, 50 c calculated in step 412 are then based on the previous velocityvector {right arrow over (v)} (ORIGIN) and the newly measured angularvelocity vector from the gyroscope 60 in the bone tracker coordinatesystem BTRK1:

{right arrow over (v)}R(LED50a)={right arrow over (ω)}(current)×{rightarrow over (x)}(ORIGIN to LED50a)

{right arrow over (v)}R(LED50b)={right arrow over (ω)}(current)×{rightarrow over (x)}(ORIGIN to LED50b)

{right arrow over (v)}R(LED50c)={right arrow over (ω)}(current)×{rightarrow over (x)}(ORIGIN to LED50c)

In step 414, velocity vectors in the localizer coordinate system LCLZcan the be calculated using the origin velocity vector {right arrow over(v)} (ORIGIN) and the relative velocity vectors {right arrow over (v)}Rof LEDs 50 a, 50 b, 50 c:

{right arrow over (v)}(LED50a)={right arrow over (v)}(ORIGIN)+{rightarrow over (v)}R(LED50a)

{right arrow over (v)}(LED50b)={right arrow over (v)}(ORIGIN)+{rightarrow over (v)}R(LED50b)

{right arrow over (v)}(LED50c)={right arrow over (v)}(ORIGIN)+{rightarrow over (v)}R(LED50c)

Steps 416 through 422 proceed the same as steps 316-322.

If the system 20 determines at step 424 that one or more of the LEDs 50a, 50 b, 50 c could not be measured during the cycle, i.e., were blockedfrom measurement, another algorithm is carried out simultaneously atsteps 500-506 shown in FIG. 7 until a complete cycle of measurements ismade where all of the LEDs 50 a, 50 b, 50 c in the cycle were visible tothe optical sensor 40. Thus, the system 20 is considered to be in a“blocked” condition until the complete cycle with all visiblemeasurements is made.

Steps 500-506 are carried out continuously while the system 20 is in theblocked condition.

In step 500 the navigation processor 52 starts a clock that tracks howlong the system 20 is in the blocked condition. The time in the blockedcondition is referred to below as t (blocked).

In step 502, the accelerometer 70 measures accelerations along the x-,y-, and z-axes of the bone tracker coordinate system BTRK1 to trackerrors in the constant velocity assumption. Accelerometer readings, likegyroscope readings are transformed from the accelerometer coordinatesystem to the bone tracker coordinate system BTRK1.

If the accelerometer 70 detects acceleration(s) that exceed predefinedacceleration tolerance(s), the navigation computer 26 will put thesystem 20 into an error condition. The acceleration tolerances could bedefined differently along each x-, y-, and z-axis, or could be the samealong each axis. If a measured acceleration exceeds a tolerance then theconstant velocity assumption is unreliable and cannot be used for thatparticular application of surgical navigation. Different tolerances maybe employed for different applications. For instance, during roboticcutting, the tolerance may be very low, but for visual navigation only,i.e., not feedback for cutting control loop, the tolerance may be sethigher.

In step 504, velocity errors associated with the positions of the LEDs50 relative to the optical sensor 40 are taken into account andmonitored during the blocked condition. For each of the LEDs 50, thevelocity error v_(error) multiplied by the time in the blocked conditiont_(blocked condition) must be less than a position error tolerance γ andthus must satisfy the following equation to prevent the system 20 frombeing put into an error condition:

v _(error) ×t _(blocked condition)<γ

In this equation, the velocity error v_(error) is calculated for each ofthe LEDs 50 a, 50 b, 50 c as follows:

$v_{error} = \frac{x_{{error}(t)} + x_{{error}({t - 1})}}{\Delta t}$

Position errors x_(error(t)) and x_(error(t-1)) are predefined positionerrors in the system 20 that are based on location relative to theoptical sensors 40 at times t and t-1. In essence, the further away theLEDs 50 a, 50 b, 50 c are located from the optical sensors 40, thehigher the potential position errors. These positions errors are derivedeither experimentally or theoretically and placed in a look-up table orformula so that at each position of the LEDs 50 a, 50 b, 50 c inCartesian coordinates (x, y, z) an associated position error isprovided.

In step 504, the localization engine 100 accesses this look-up table orcalculates this formula to determine the position errors for each ofLEDs 50 a, 50 b, 50 c at the current time t and at the previous timet-1. The position errors are thus based on the positions in Cartesiancoordinates in the localizer coordinate system LCLZ calculated by thesystem 20 in step 422 for the current time t and at the previous timet-1. The time variable Δt represents the time it takes for subsequentposition calculations, so the difference between t and t-1, which forillustrative purposes may be 1 millisecond.

The position error tolerance γ is predefined in the navigation computer26 for access by the localization engine 100. The position errortolerance γ could be expressed in millimeters. The position errortolerance γ can range from 0.001 to 1 millimeters and in someembodiments is specifically set at 0.5 millimeters. Thus, if theposition error tolerance γ is set to 0.5 millimeters, the followingequation must be satisfied:

v _(error) ×t _(blocked condition)<0.5 millimeters

As can be seen, the longer the system 20 is in the blocked condition,the larger the effect that the time variable has in this equation andthus the smaller the velocity errors that will be tolerated. In someembodiments, this equation is calculated by the localization engine 100in step 504 separately for each of the LEDs 50 a, 50 b, 50 c. In otherembodiments, because of how closely arranged the LEDs 50 a, 50 b, 50 care on the tracker 44, the velocity error of only one of the LEDs 50 a,50 b, 50 c is used in this calculation to determine compliance.

In step 506, when the error(s) exceeds the position error tolerance y,the system 20 is placed in an error condition. In such a condition, forexample, any control or movement of cutting or ablation tools is ceasedand the tools are shut down.

IV. Other Embodiments

In one embodiment, when each of the trackers 44, 46, 48 are beingactively tracked, the firing of the LEDs occurs such that one LED fromtracker 44 is fired, then one LED from tracker 46, then one LED fromtracker 48, then a second LED from tracker 44, then a second LED fromtracker 46, and so on until all LEDs have been fired and then thesequence repeats. This order of firing may occur through instructionsignals sent from the transceivers (not shown) on the camera unit 36 totransceivers (not shown) on the trackers 44, 46, 48.

The navigation system 20 can be used in a closed loop manner to controlsurgical procedures carried out by surgical cutting instruments. Boththe instrument 22 and the anatomy being cut are outfitted with trackers50 such that the navigation system 20 can track the position andorientation of the instrument 22 and the anatomy being cut, such asbone.

In one embodiment, the navigation system is part of a robotic surgicalsystem for treating tissue. In some versions, the robotic surgicalsystem is a robotic surgical cutting system for cutting away materialfrom a patient's anatomy, such as bone or soft tissue. The cuttingsystem could be used to prepare bone for surgical implants such as hipand knee implants, including unicompartmental, bicompartmental, or totalknee implants. Some of these types of implants are shown in U.S. patentapplication Ser. No. 13/530,927, entitled, “Prosthetic Implant andMethod of Implantation”, the disclosure of which is hereby incorporatedby reference.

The robotic surgical cutting system includes a manipulator (see, forinstance, FIG. 1 ). The manipulator has a plurality of arms and acutting tool carried by at least one of said plurality of arms. Arobotic control system controls or constrains movement of the cuttingtool in at least 5 degrees of freedom. An example of such a manipulatorand control system are shown in U.S. Provisional Patent Application No.61/679,258, entitled, “Surgical Manipulator Capable of Controlling aSurgical Instrument in either a Semi-Autonomous Mode or a Manual,Boundary Constrained Mode”, hereby incorporated by reference, and alsoin U.S. patent application Ser. No. 13/958,834, entitled, “NavigationSystem for use with a Surgical Manipulator Operable in Manual orSemi-Autonomous Mode”, the disclosure of which is hereby incorporated byreference.

In this embodiment, the navigation system 20 communicates with therobotic control system (which can include the manipulator controller54). The navigation system 20 communicates position and/or orientationdata to said robotic control system. The position and/or orientationdata is indicative of a position and/or orientation of instrument 22relative to the anatomy. This communication provides closed loop controlto control cutting of the anatomy such that the cutting occurs within apredefined boundary.

In this embodiment, manipulator movement may coincide with LEDmeasurements such that for each LED measurement taken, there is acorresponding movement of the instrument 22 by the manipulator 56.However, this may not always be the case. For instance, there may besuch a lag between the last LED measurement and movement by themanipulator 56 that the position and/or orientation data sent from thenavigation computer 26 to the manipulator 56 for purposes of controlloop movement becomes unreliable. In such a case, the navigationcomputer 26 can be configured to also transmit to the manipulatorcontroller 54 kinematic data. Such kinematic data includes thepreviously determined linear and angular velocities for the trackers 44,46, 48. Since the velocities are already known, positions can calculatedbased on the lag of time. The manipulator controller 54 could thencalculate, for purposes of controlling movement of the manipulator 56,the positions and orientations of the trackers 44, 46, 48 and thus, therelative positions and orientations of the instrument 22 (or instrumenttip) to the femur F and/or tibia T.

In this embodiment, the instrument 22 is held by the manipulator shownin FIG. 1 or other robot that provides some form of mechanicalconstraint to movement. This constraint limits the movement of theinstrument 22 to within a predefined boundary. If the instrument 22strays beyond the predefined boundary, a control is sent to theinstrument 22 to stop cutting.

When tracking both the instrument 22 and the anatomy being cut in realtime in these systems, the need to rigidly fix anatomy in position canbe eliminated. Since both the instrument 22 and anatomy are tracked,control of the instrument 22 can be adjusted based on relative positionand/or orientation of the instrument 22 to the anatomy. Also,representations of the instrument 22 and anatomy on the display can moverelative to one another—to emulate their real world motion.

In one embodiment, each of the femur F and tibia T has a target volumeof material that is to be removed by the working end of the surgicalinstrument 22. The target volumes are defined by one or more boundaries.The boundaries define the surfaces of the bone that should remain afterthe procedure. In some embodiments, system 20 tracks and controls thesurgical instrument 22 to ensure that working end, e.g., bur, onlyremoves the target volume of material and does not extend beyond theboundary, as disclosed in Provisional Patent Application No. 61/679,258,entitled, “Surgical Manipulator Capable of Controlling a SurgicalInstrument in either a Semi-Autonomous Mode or a Manual, BoundaryConstrained Mode”, hereby incorporated by reference.

In the described embodiment, control of the instrument 22 isaccomplished by utilizing the data generated by the coordinatetransformer 102 that indicates the position and orientation of the buror other cutting tool relative to the target volume. By knowing theserelative positions, the surgical instrument 22 or the manipulator towhich it is mounted, can be controlled so that only desired material isremoved.

In other systems, the instrument 22 has a cutting tool that is movablein three degrees of freedom relative to a handheld housing and ismanually positioned by the hand of the surgeon, without the aid ofcutting jig, guide arm or other constraining mechanism. Such systems areshown in U.S. Provisional Patent Application No. 61/662,070, entitled,“Surgical Instrument Including Housing, a Cutting Accessory that Extendsfrom the Housing and Actuators that Establish the Position of theCutting Accessory Relative to the Housing”, the disclosure of which ishereby incorporated by reference.

In these embodiments, the system includes a hand held surgical cuttinginstrument having a cutting tool. A control system controls movement ofthe cutting tool in at least 3 degrees of freedom using internalactuators/motors, as shown in U.S. Provisional Patent Application No.61/662,070, entitled, “Surgical Instrument Including Housing, a CuttingAccessory that Extends from the Housing and Actuators that Establish thePosition of the Cutting Accessory Relative to the Housing”, thedisclosure of which is hereby incorporated by reference. The navigationsystem 20 communicates with the control system. One tracker (such astracker 48) is mounted to the instrument. Other trackers (such astrackers 44, 46) are mounted to a patient's anatomy.

In this embodiment, the navigation system 20 communicates with thecontrol system of the hand held surgical cutting instrument. Thenavigation system 20 communicates position and/or orientation data tothe control system. The position and/or orientation data is indicativeof a position and/or orientation of the instrument 22 relative to theanatomy. This communication provides closed loop control to controlcutting of the anatomy such that the cutting occurs within a predefinedboundary (the term predefined boundary is understood to includepredefined trajectory, volume, line, other shapes or geometric forms,and the like).

Features of the invention may be used to track sudden or unexpectedmovements of the localizer coordinate system LCLZ, as may occur when thecamera unit 36 is bumped by surgical personnel. An accelerometer (notshown) mounted to camera unit 36 monitors bumps and stops system 20 if abump is detected. In this embodiment, the accelerometer communicateswith the camera controller 42 and if the measured acceleration along anyof the x, y, or z axes exceeds a predetermined value, then the cameracontroller 42 sends a corresponding signal to the navigation computer 26to disable the system 20 and await the camera unit 36 to stabilize andresume measurements. In some cases, the initialization step 200, 300,400 would have to be repeated before resuming navigation.

In some embodiments, a virtual LED is positioned at the working tip ofthe instrument 22. In this embodiment, the virtual LED is located at thelocation of the working tip in the model of the instrument tracker 48 sothat the working tip location is continuously calculated.

It is an object of the intended claims to cover all such modificationsand variations that come within the true spirit and scope of thisinvention. Furthermore, the embodiments described above are related tomedical applications, but the inventions described herein are alsoapplicable to other applications such as industrial, aerospace, defense,and the like.

What is claimed is:
 1. A surgical system, comprising: a roboticmanipulator; an instrument coupled to the robotic manipulator, whereinthe robotic manipulator is configured to control movement of theinstrument to facilitate a surgical procedure; a non-optical sensorcoupled to the instrument; and a navigation system configured obtainreadings from the non-optical sensor, and the navigation systemcomprising a camera unit configured to optically track a pose of theinstrument, wherein the navigation system is configured to: detect acondition whereby the camera unit is blocked from optically tracking thepose of the instrument; and in response to detection of the condition,track the pose of the instrument with the readings from the non-opticalsensor.
 2. The surgical system of claim 1, wherein the navigation systemis configured to: detect that the camera unit is no longer blocked fromoptically tracking the pose of the instrument; and in response,optically track the pose of the instrument with the camera unit.
 3. Thesurgical system of claim 1, wherein, in response to detection of thecondition, the navigation system is configured to utilize readings fromthe non-optical sensor to estimate the pose of the instrument.
 4. Thesurgical system of claim 1, wherein, in response to detection of thecondition, the navigation system tracks the pose of the instrument byfurther being configured to: obtain a previous pose of the instrumentthat was measured based on optically tracking the pose of the instrumentby the camera unit prior to detection of the condition; and utilizereadings from the non-optical sensor to calculate a current pose of theinstrument relative to the previous pose.
 5. The surgical system ofclaim 1, wherein the non-optical sensor comprises an accelerometer. 6.The surgical system of claim 1, wherein the non-optical sensor comprisesa gyroscope.
 7. The surgical system of claim 1, wherein the roboticmanipulator is configured to utilize the instrument to prepare a patientanatomy to receive an implant.
 8. The surgical system of claim 1,wherein the robotic manipulator comprises a robotic arm that issupported by a moveable cart.
 9. The surgical system of claim 1,wherein, in response to detection of the condition, the navigationsystem is configured to communicate the tracked pose of the instrumentto a control system coupled to the robotic manipulator to enable therobotic manipulator to control or constrain movement of the instrumentrelative to a predefined path or boundary.
 10. A navigation systemconfigured to be utilized with a robotic manipulator configured tocontrol movement of an instrument to facilitate a surgical procedure,the navigation system comprising: a camera unit configured to opticallytrack a pose of the instrument; a non-optical sensor coupled to theinstrument; and a computing system coupled to the camera unit and beingconfigured to obtain readings from the non-optical sensor, and thecomputing system being configured to: detect a condition whereby thecamera unit is blocked from optically tracking the pose of theinstrument; and in response to detection of the condition, track thepose of the instrument with the readings from the non-optical sensor.11. The navigation system of claim 10, wherein the computing system isconfigured to: detect that the camera unit is no longer blocked fromoptically tracking the pose of the instrument; and in response,optically track the pose of the instrument with the camera unit.
 12. Thenavigation system of claim 10, wherein, in response to detection of thecondition, the computing system is configured to utilize readings fromthe non-optical sensor to estimate the pose of the instrument.
 13. Thenavigation system of claim 10, wherein, in response to detection of thecondition, the computing system tracks the pose of the instrument byfurther being configured to: obtain a previous pose of the instrumentthat was measured based on optically tracking the pose of the instrumentby the camera unit prior to detection of the condition; and utilizereadings from the non-optical sensor to calculate a current pose of theinstrument relative to the previous pose.
 14. The navigation system ofclaim 10, wherein the non-optical sensor comprises an accelerometer. 15.The navigation system of claim 10, wherein the non-optical sensorcomprises a gyroscope.
 16. The navigation system of claim 10, wherein,in response to detection of the condition, the computing system isconfigured to communicate the tracked pose of the instrument to acontrol system coupled to the robotic manipulator to enable the roboticmanipulator to control or constrain movement of the instrument relativeto a predefined path or boundary.
 17. A method of operating a navigationsystem including a camera unit, a non-optical sensor, and a computingsystem, the non-optical sensor being coupled to an instrument that iscontrolled by a robotic manipulator, the method comprising: opticallytracking a pose of the instrument with the camera unit; detecting, withthe computing system, a condition whereby the camera unit is blockedfrom optically tracking the pose of the instrument; obtaining, with thecomputing system, readings from the non-optical sensor; and in responseto detecting the condition, the computing system tracking the pose ofthe instrument with the readings from the non-optical sensor.
 18. Themethod of claim 17, comprising: detecting, with the computing system,that the camera unit is no longer blocked from optically tracking thepose of the instrument; and in response, optically tracking the pose ofthe instrument with the camera unit.
 19. The method of claim 17,comprising: in response to detecting the condition, the computing systemutilizing readings from the non-optical sensor for estimating the poseof the instrument.
 20. The method of claim 17, comprising, in responseto detecting the condition, the computing system tracking the pose ofthe instrument by further: obtaining a previous pose of the instrumentthat was measured based on optically tracking the pose of the instrumentby the camera unit prior to detecting the condition; and utilizingreadings from the non-optical sensor for calculating a current pose ofthe instrument relative to the previous pose.